Deformation of the Northwestern Okhotsk Plate: How is it … · 2020. 6. 2. · seismicity (Fig....

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Deformation of the Northwestern Okhotsk Plate:How is it happening?

D. Hindle1, K. Fujita 2, and K. Mackey2

1Albert-Ludwigs-Universitat Freiburg, Geologisches Institut, Albertstr. 23b, 79104, Freiburg i. Br., Germany2Dept. Geol. Sci., Michigan State University, East Lansing, MI 48824-1115, USA

Abstract. The Eurasia (EU) – North America (NA) plateboundary zone across Northeast Asia still presents manyopen questions within the plate tectonic paradigm. Con-straining the geometry and number of plates or microplatespresent in the plate boundary zone is especially difficult be-cause of the location of the EU-NA euler pole close to oreven upon the EU-NA boundary. One of the major chal-lenges remains the geometry of the Okhotsk plate (OK).whose northwestern portion terminates on the EU-OK-NAtriple junction and is thus caught and compressed betweenconverging EU and NA. We suggest that this leads to a co-herent and understandable large scale deformation pattern ofmostly northwest-southeast trending strike-slip faults whichsplit Northwest OK into several extruding slivers. When thefault geometry is analysed together with space geodetic andfocal mechanism data it suggests a central block which is ex-truding faster bordered east and west by progressively slowerextruding blocks until the OK plate boundary faults are en-countered. Taking into account elastic loading from both theintra-OK faults and the OK-Pacific (PA) boundary reconcilesgeodetic motions with geologic slip rates on at least the OK-NA boundary which corresponds to the Ulakhan fault.

1 Introduction

Northeast Russia (Fig. 1) has for a long time been identi-fied as a region where both the number and geometry oftectonic plates and microplates are subject to uncertainty(Morgan, 1968; Seno et al., 1996; Stein and Sella, 2002).This is mostly due to the fact that the EU-NA rotation polelies very close to their mutual plate boundary (e.g.,Sellaet al., 2002) with a gradual transition from active spread-ing along the Mid-Atlantic and Arctic (Gakkel) ridges (El-dholm et al., 1990) to mostly transpression further south in

Correspondence to:D. Hindle([email protected])

the Central Chersky Range (Figs. 1 and 2) (Fujita et al.,2009, this volume). As a result, a nutcracker type mo-tion affects a roughly triangular shaped zone including theSuntar-Khayata, Kukhtuy and Central Chersky Ranges andextending southwards to the northern coastline of the Sea ofOkhotsk (Figs. 1 and 2). It may also lead to varying degreesof “extrusion” of this region towards the south or southeastas it is squeezed between the two giant plates. This segmentof the complex EU-NA plate boundary zone has often beenconsidered part of the Okhotsk plate (OK) (Cook et al., 1986;Riegel et al., 1993; Seno et al., 1996; Fujita et al., 1997; Hin-dle et al., 2006), which extends east to the Kamchatka-Kurilsubduction margin, west to Sakhalin Island and south to theNorthern Sea of Japan, though historically, a range of alter-native configurations of plates, blocks and microplates havebeen suggested for the region (Fujita et al., 2009, this vol-ume).

The rigidity of OK, particularly its northern landward por-tion extending from the northern coastline of the Sea ofOkhotsk to the apex formed by the EU-OK-NA triple junc-tion, will strongly influence the kinematics of partitioning ofEU-NA convergence. The northwestern portion of OK dealtwith in this paper forms part of the Chersky Seismic Belt(CSB) of diffuse seismicity (Figs. 1 and 2) (Parfenov et al.,1988; Riegel et al., 1993). Seismicity of the CSB includingthe landward portion of OK extending from Magadan andOkhotsk on the northern coast of the Sea of Okhotsk, to thepresumed EU-OK-NA triple junction is described in detailby Fujita et al.(2009, this volume). It is therefore impor-tant to understand whether the diffuse seismicity of the CSBrepresents an even, continuous distribution of EU-NA con-vergence across Northwestern OK or whether more localisedslip, possibly associated with occasional large earthquakes,on larger structures, controls the kinematics of the region.

A series of end member, kinematic models for deforma-tion partitioning across OK (Hindle et al., 2006) tested sev-eral mechanical scenarios for a distinct, Northwestern OKregion undergoing compressional deformation and possibleextrusion between EU and NA. The models considered the

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148 D. Hindle et al.: Deformation Northwestern OkhotskFigures

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Fig. 1. Seismicity of northeast Asia and inferred plate boundaries (heavy black lines). Major focal mechanisms,

scaled for magnitude (largest event is the Mw6.4 Artyk earthquake of 1971), small earthquake epicentres (light

grey dots), GPS velocities of Steblov et al. (2003) referenced to fixed NA, with 1σ confidence ellipses (black

arrows) and station names, and EU-NA motion vectors calculated at each GPS site based on the EU-NA euler

vector of Sella et al. (2002) (grey shaded arrows) are also shown

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Fig. 1. Seismicity of Northeast Asia and inferred plate boundaries (heavy black lines). Major focal mechanisms, scaled for magnitude(largest event is theMw 6.4 Artyk earthquake of 1971), small earthquake epicentres (light grey dots), GPS velocities ofSteblov et al.(2003)referenced to fixed NA, with 1σ confidence ellipses (black arrows) and station names, and EU-NA motion vectors calculated at each GPSsite based on the EU-NA euler vector ofSella et al.(2002) (grey shaded arrows) are also shown.

northwestern portion of OK to be either an unbroken orsmoothly deforming piece of lithosphere, and ignored spe-cific geological details of the structure of the plate. In thispaper, we examine in more detail the issue of how the north-western corner of OK is deforming, and attempt to directlyintegrate geological and earthquake data into a discrete, kine-matic model of this portion of the CSB.

2 Tectonics of Northwestern Okhotsk

2.1 Boundaries of Northwestern Okhotsk

In this paper, the geometry of Northwestern OK is definedfollowing the plate boundaries suggested byFujita et al.(1997). The OK-NA boundary is traced along the Ulakhanfault, the largest fault in the Central Chersky ranges, visi-ble in satellite images over a distance of∼1000 km (Par-fenov et al., 1988; Imaev et al., 1994; Fujita et al., 2009)

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D. Hindle et al.: Deformation Northwestern Okhotsk 149

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lines), major focal mechanisms, scaled for magnitude (including the Mw7.0 Neftegorsk earthquake and the

Mw6.4 Artyk earthquake), small earthquake epicentres (red dots), GPS velocities of Steblov et al. (2003)

referenced to fixed NA (blue arrows) and major lineaments identified within OK. Yellow boxes delimit the

approximate positions of figures 5a-d.

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Fig. 2. Present day tectonics of the EU-NA-OK system. Figure shows major plate boundaries (heavy black lines), major focal mechanisms,scaled for magnitude (including theMw 7.0 Neftegorsk earthquake and theMw 6.4 Artyk earthquake), small earthquake epicentres (reddots), GPS velocities ofSteblov et al.(2003) referenced to fixed NA (blue arrows) and major lineaments identified within OK. Yellow boxesdelimit the approximate positions of Fig. 5a–d.

and associated with teleseismic earthquakes M4-5 and localseismicity (Fig. 2). Northeast of the Ulakhan fault, there is anoticeable drop in seismicity (Figs. 1 and 2), and the faultcuts and offsets post-Pliocene drainage networks (Krylov,1971; Fujita et al., 2004) and Recent alluvial fans. Furthereast, in the northern segment of the Kamchatka peninsula,uplift of the Ozernoi peninsula (Fig. 1) suggesting consider-able (∼20–30 mm/yr) convergence rates has been explainedby the meeting of a slowly rotating Bering Block (aseismicKomandorskii basin) (Mackey et al., 1997) and a short, ex-truding segment of OK (Pedoja et al., 2006). This interpreta-tion further supports a throughgoing plate boundary betweenOK and NA, extending from the end of the traceable Ulakhanfault, eastwards across Shelikov Bay (Fig. 2), and into North-ern Kamchatka (Fujita et al., 2009, this volume).

The OK-EU boundary is marked by a system of north-south striking faults (Ketanda system) which crosses the CSBnorth and west of the settlement of Okhotsk (Figs. 1 and2). The faults are associated with earthquakes with poorlyconstrained focal mechanisms indicating mostly right lateraltranspression (Fujita et al., 2009, this volume). The Ke-tanda system reaches the Sea of Okhotsk slightly west ofthe settlement of Okhotsk, almost directly north of Sakhalin

Island and the site of theMw 7.0 Neftegorsk earthquake(Fig. 2) which occurred on a NNE-SSW striking, verti-cal fault considered to be evidence of a plate boundaryin at least the northern part of Sakhalin Island (Fournieret al., 1994; Seno et al., 1996). Transpressional flowerstructures are known from the sea floor south of Okhotskin the North Schmidt Deformation Zone/Kashevarov fault(Worrall et al., 1996) (Fig. 2) but there is an absence of seis-micity along this segment of the potential plate boundary(Fujita et al., 2009, this volume), except for aMw 4.8, thrustevent (11 September 2006) north of Sakhalin Island and closeto the presumed trace of the boundary. This, together with therelatively continuous distribution of microseismicity west-wards, across the presumed trace of the EU-OK boundaryin the landward region of the Ketanda system, make its defi-nition between Northern Sakhalin and the potential EU-OK-NA triple junction more difficult. Nevertheless, seismic qui-escence due to periodic locking is known along other plateboundary segments, for instance portions of the San Andreassystem. It is also possible that the western boundary of OKis undergoing stable creep, perhaps enhanced by a warmerlithosphere suggested beneath the Northern Sea of Okhotskfrom tomographic studies (Levin et al., 2002).

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150 D. Hindle et al.: Deformation Northwestern Okhotsk

Table 1. Strain rates calculated for the apex region of the Okhotskplate, (fromHindle et al., 2006), giving contraction, extension mag-nitudes, degrees clockwise from north of contraction axis and a ver-tical thickening parameter calculated to conserve volume. Confi-dence limits for approximate 1σ variations of EU-NA pole and an-gular velocity.

Data contraction extension cont. azi. vert. thick.yr−1 yr−1 ◦ from N. yr−1

modelcontr. −5.8±1 e−9 −4.9±3 e−10 62±1 6.3±1.4 e−9smo. ext. −3.5±1 e−9 3.0±0.6 e−9 62±1 –rig. ext. – – – - -

obs.GPS −3.7 e−9 6.1 e−9 85 −2.1 e-9Eq. −3.8 e−10 3.5 e−10 81 2.6 e-11

2.1.1 Active deformation

The northwestern segment of OK is a zone of diffuse seis-micity (Figs. 1 and 2). Focal mechanisms of varying reliabil-ity have been determined for a number of larger earthquakes(>Mw 5) and are discussed in detail inFujita et al.(2009,this volume). The largest recorded event from the region, the1971Mw 6.4 Artyk event (Imaev et al., 2000; Fujita et al.,2002), occurred on the Chai-Yureya fault, a major structurecutting through Northwestern OK (Figs. 2 and 5) (Imaev etal., 1994). Other events with determined focal mechanismsoccur on or near to several other such structures. A com-bination of published moment tensor solutions and reliablefocal mechanisms and magnitudes confined to within North-western OK were summed to estimate a seismic strain rateacross the region byHindle et al.(2006) (see Table 1). Thisgives contractional strain rates−3.8×10−10 yr −1, directed∼081◦ as the largest principle seismic strain rate component(Fig. 3a).

As well as the seismic record from Northwestern OK, GPSvelocities give a measure of internal strain across the re-gion (Kogan et al., 2003; Steblov et al., 2003). Hindle etal. (2006) used GPS stations best able to represent strainacross Northwestern OK between EU and NA to calculatea geodetic strain rate for the region. This shows uniquely ex-tensional strain rates with largest extensional principal strainrate (6.1×10−9 yr−1 directed∼175◦), with subordinate con-traction (−3.7×10−9 yr−1 directed∼085◦) (Fig. 3b). Thesedata are based on velocity vectors uncorrected for transient,elastic displacements due to locking on either plate bound-ary faults or large faults within Northwestern OK itself. Ithas also been noted that elastic displacement due to lockingof the Kamchatka subduction margin (Burgmann et al., 2005)may subtract as much as∼1–2.5 mm/yr of motion from thesouthernmost stations in Northwestern OK.

2.1.2 Deformation models

Geometric deformation models, based on EU-NA conver-gence velocities along the boundaries of OK derived from theEU-NA euler vector ofSella et al.(2002) were constructedby Hindle et al.(2006). Three end-member modes of parti-tioning EU-NA convergence across Northwestern OK weretested (Fig. 3c–e, Table 1).

A pure contraction mode (Fig. 3c) where all convergenceis consumed within OK by velocity gradients parallel tosmall circle tracks around the EU-NA pole and no material“escapes” southward yields shortening orientations similar toseismic and GPS strain, (−5.8×10−9 yr −1, directed∼062◦).

A smooth extrusion model (Fig. 3d) where the surfacearea of the deforming region is conserved by allowingevenly distributed area gain on the southern border of themodel to compensate for area loss due to EU-NA conver-gence, thus modifying the pure contraction velocity fieldand adding a mostly south directed extrusion componentgives strain rates comparable to the pure contraction sce-nario (−3.5×10−9 yr −1, directed∼062◦). This model ve-locity field also has a velocity gradient (increasing south-wards) along the Ulakhan fault (identified as a candidate OK-NA plate boundary). Maximum slip resolved in the directionof the Ulakhan fault is∼3.0 mm/yr.

A rigid extrusion model (Fig. 3e) where Northwestern OKextrudes as a rigid whole, along the Ulakhan fault, by anamount sufficient to “fit” at the closing, southern end hasno internal strain of OK though a gap (fault) is left fromthe triple junction southwards along the western boundaryof OK. This configuration gives a constant slip rate along theUlakhan fault of∼5.5 mm/yr.

3 Block model for Northwestern Okhotsk

The analysis ofHindle et al.(2006) did not identify a pre-ferred mode of accomodating EU-NA convergence acrossNorthwestern OK, but rather suggested that a combinationof all 3 end member models might be operating. Thus, amore detailed consideration of geological evidence for de-formation of Northwestern OK is useful.

Russian workers first suggested the presence of a largenumber of linear tectonic features across the region, iden-tifiable from a combination of satellite images, present dayseismicity, geomorphology and other field data (Imaev et al.,1994; Parfenov et al., 1988). These were suggested to forma network of mostly strike-slip faults with variable senses ofslip. A new interpretation of the fault network, based on to-pographic data, satellite images, seismicity and geomorphol-ogy (offset drainagesFujita et al., 2004) is shown in Fig. 4,where some of the major structures have been assumed toform fault-boundaries of discrete, tectonic blocks. These di-vide Northwestern OK into six, fault-bounded, regions withleft lateral slip on block boundaries 2/3 and 3/4 and right



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Fig. 3. Observed and modelled deformation rates across Northwestern OK (for details see Hindle et al., 2006).(a) Moment tensor sumshowing focal mechanisms and strain rate axes used in calculations(b) Averaged GPS strain rate calculated across the grey shaded polygon,based on velocities ofSteblov et al.(2003), vectors shown as black arrows(c) Pure contraction model where velocities (shown as blackarrows at selected points) are parallel to and decrease linearly along small circle tracks around the EU-NA pole ofSella et al.(2002) (d)Smooth extrusion model with additional extrusion component to allow area conservation of OK added to contraction vectors (black arrows).(e)Rigid extrusion model, with constant velocities (black arrows) across an extruding Northwestern OK block.

lateral slip on block boundaries 4/5 and 5/6 and probably onpart of 1/2 (Darpir) (Fig. 4). The relative slip senses implya gradient of southward extrusion across the region, with themaximum rate reached in the central block, 4, and decreasingfrom there to each plate boundary.

It is interesting to analyse the hypothetical block geometryshown in Fig. 4 which coincides with part of NorthwesternOK, in the light of both focal mechanisms and microseismic-ity and the recent geodetic velocity field. The 1971 ArtykMw 6.4 earthquake and aftershock sequence (Fujita et al.,2002) (see alsoFujita et al., 2009, this volume, Fig. 13) ap-pear certain to have occurred on the Chai-Yureya fault andmark the largest recorded release of seismic energy withinOK, including its presumed boundary along the Ulakhanfault. The event had a left-lateral mechanism. Offset of smallrivers by up to∼1 km has been noted along the Chai-Yureyafault, as well as an offset of∼8 km of a Late Jurassic dolerite.The Chai-Yureya fault forms the boundary between blocks 3

and 4 in our model (Fig. 4). Block 3 contains GPS stationsSUS1 and TAL1, and block 4 contains KUL1 (Fig. 1) andGPS velocities suggest slight, left-lateral motion betweenthe two blocks, although based on sparse data. There isalso a north-to-south, increase in velocity within block 3.This may reflect transient loading and differential locking onthe block 2/3 boundary fault, and hence, does not excludea tectonically intact, rigid block 3. The block 2/3 bound-ary fault also has associated left-lateral focal mechanisms onor near to it. Block 1, bounded by the Darpir and Ulakhanfaults, has an assumed right-lateral sense on its border withblock 2, however this is not confirmed by seismic or geode-tic observations. The Ulakhan fault itself forms a left-lateralboundary with all blocks bordering it, supported by evi-dence on a range of timescales, from post-Pliocene offset ofdrainage networks (this volumeFujita et al., 2004, 2009, todisplacement of feeder channels in Recent alluvial fans (e.g.Seymchan-Buyunda fan) and also some earthquake focalmechanisms. The block 4/5 boundary formed by the Elgin-Oymyakon fault system was suggested to be left-lateral by



Artyk, 1971, Mw 6.4left lateral

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Fig. 4. Network of tectonic lineaments (mostly strike-slip faults) in northwestern OK (after Imaev et al. (1994)).

Slip senses on major faults (black paired arrows) are given where possible to correspond to focal mechanisms,

but otherwise follow Imaev et al. (1994). GPS vectors (red arrows) of Steblov et al. (2003) are also shown. The

lineaments (red lines) have been used to divide northwestern OK into 6 distinct blocks, mostly terminating on

the Inya-Yama fault, in the south.

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Fig. 4. Network of tectonic lineaments (mostly strike-slip faults) in Northwestern OK (afterImaev et al., 1994). Slip senses on major faults(black paired arrows) are given where possible to correspond to focal mechanisms, but otherwise followImaev et al.(1994). GPS vectors(red arrows) ofSteblov et al.(2003) are also shown. The lineaments (red lines) have been used to divide Northwestern OK into 6 distinctblocks, mostly terminating on the Inya-Yama fault, in the south.

Imaev et al.(1994), but is associated with a right-lateral fo-cal mechanism (Fig. 4). Only one GPS station is locatedwithin block 5 (TOM1) in its northernmost part, near theEU-OK-NA triple junction and trends almost exactly as thelinear velocity from the EU-NA euler vector at this point(Fig. 1). Given the location of the GPS station in a com-plex region near several boundaries and a triple junction, thevelocity measured may be transient and influenced by inter-ference between elastic loading along several locked faultsand boundaries.

The southern termination of the block model poses an in-teresting problem. The region bounded by Inya-Yama faultand the Chelomdzha-Yama fault (Fig. 4) may mark a dividebetween a more rigid, core region of OK and a softer regionof warm lithosphere beneath the Sea of Okhotsk (Levin et al.,2002). The Chelomdzha-Yama fault (Fig. 4), an east-westtrending lineament near the coast, has been considered a left-lateral, transpressional lineament by some Russian workers(Imaev et al., 1994). However, an apparently left-lateral fo-cal mechanism near the trace of the lineament whose possi-

ble slip vectors do not line up particularly well with the mainfault trend is now thought to be associated with a subordinate,WNW trending feature (Fujita et al., 2009, this volume). Thestructural history of the southern onshore and offshore sea ofOkhotsk region is one of Tertiary age, NNW-SSE extension(e.g.,Worrall et al., 1996), and east-west trending lineamentsare probable basin bounding faults from this period. Cur-rently, limited GPS measurements suggest slight (∼1 mm/yr)convergence between the fastest, southward escaping regionof Northern OK within block 4, and the coastal GPS stationof Magadan, meaning that the southern region delimited bythe Inya-Yama and Chelomdzha-Yama faults is experiencingslight,∼N-S contraction, which may only have begun fairlyrecently. Thus, it appears that the extrusion of Northwest-ern OK does not end entirely before the coast of Okhotsk,but that it is most probably continued southwards into theaseismic Northern Sea of Okhotsk, which may be composedof warmer lithosphere undergoing ductile deformation, be-hind the still eastward retreating Kamchatka-Kuril arc. Also,we note that due to the apparently very low N-S shortening



rates and the short (∼5 Ma–10 Ma) time over which short-ening is likely to have occurred, only a small amount (∼1–2 km) of contractional deformation is likely to have accumu-lated across the southern region of Northwestern OK, whichwould remain imperceptible if spread out across many smallstructures across the region’s width (∼200 km in a N-S di-rection).

The region bordering the proposed EU-OK-NA triplejunction is also not defined as a block or group of blocks.Seismic and geodetic data from this region are neverthelessinteresting. The northernmost GPS station (UNR1) showsmotion directed very close to the trend of the linear velocitypredicted by the EU-NA euler vector ofSella et al.(2002),however, it is of greater magnitude (Fig. 1). The northern-most vector within block 5 (TOM1) is similarly directed,as already discussed. One focal mechanism, located on thenorthern tip of a branch of the Chai-Yureya fault gives a purethrust solution. The combination of GPS data and seismicityin this region suggests that it may be a local manifestationof the pure contraction deformation mode ofHindle et al.(2006).

Figure 5a–d shows some examples of important Neogenestructures that can be traced on satellite images. Figure 5ashows the Inya-Yama fault, which is the lineament that hasthe clearest expression in satellite images within OK. Thislineament is not clearly associated with any earthquakes, andits linearity suggests that if it is active, it is a strike-slipfault. No clear linear feature is noticeable along the traceof the Chai-Yureya fault (Fig. 5b) although the Upper Nerabasin appears to have opened on a releasing bend along thefault. The Ketanda fault (Fig. 5c) is associated with earth-quakes (seeFujita et al., 2009, this volume, but its course isalso not easy to distinguish from satellite images. Finally,Fig. 5d shows the southern end of the Elgin-Oymyakon sys-tem, where there may be some morphological evidence of abasin opening. In general, we note that the lack of clear satel-lite expression of faults may not to be a criteria for ruling outthe presence of active faults as is demonstrated in particularby the Chai-Yureya fault, which is seismically active today,has traceable offsets of probably recent river systems, but re-mains morphologically indistinguishable.

4 Model and GPS deformation fields and faultorientations

In the light of the block model analysis and also the networkof apparently large strike-slip faults cutting through North-western OK, it is interesting to analyse the strain fields sug-gested by both observation and models and compare theseto fault orientations. FollowingHolt (2000) who comparedmantle anisotropy from shear wave splitting (Silver, 1996)to surface deformation measured by geodesy in Tibet, Fig. 6shows two calculations of surface deformation fields (shearplanes), based on the “smooth extrusion” model for OK

(Hindle et al., 2006) (Fig. 3d) and GPS velocities (Steblov etal., 2003). These planes are the lines of no length change insub-regions of the two velocity fields. We compare the shearplanes here to surface deformation suggested by the orien-tations of active faults (block boundaries). For the smoothextrusion model, one or both shear planes appear to matchclosely the directions of the major faults, suggesting thatthere may be some compatibility between the geometricallyderived velocity field and the longer term strain being ac-cumulated within Northwestern OK. The GPS derived shearplanes fit less well, perhaps because this field represents aninstantaneous, elastic signal of deformation that requires fil-tering to recover the permanent deformation it will lead to(Apel et al., 2006).

5 Discussion

Given the combination of evidence from GPS geodesy, seis-micity and geology, a block model to explain the deformationof Northwestern OK is reasonable. Nevertheless, several is-sues need careful consideration. Geodetic evidence is keyto establishing the relative motion of potential blocks. How-ever, there are important transient, elastic components in thissignal due to locking on the faults and elastic strain accu-mulation. The obvious solution to this would be a longertimespan of observations, presumably over an entire seismiccycle, which is itself an unknown quantity. Without this in-formation, joint consideration of seismicity and geologicalevidence together with GPS data may give more confidencein interpreting the GPS signal. The fact that several of therelatively few focal mechanisms determined on the faults fitwith the suggested relative motions from GPS may reflectthat transient, elastic displacements effectively follow per-manent deformation on time scales of many seismic cycles.Such behaviour has been suggested in the Andean foreland(Hindle et al., 2002; Allmendinger et al., 2005).

Our block model also leads to important questions aboutthe plate boundary itself, particularly OK-NA formed by theUlakhan fault. It is currently believed that the Ulakhan faulthas longer term (∼3–5 Ma) slip rates of∼5 mm/yr (Hindle etal., 2006; Fujita et al., 2009, this volume). The block modelsuggests a complex division of slip within Northwestern OKwith a fastest extruding central region, not directly border-ing NA. The magnitude of the largest geodetic displacement(KUL1, within block 4) relative to fixed NA is∼4.3 mm/yr,whilst the TAL1 station (block 3) has a velocity magnitudeof ∼3 mm/yr relative to fixed NA. There are thus potentialdiscerepancies between measured displacements and the as-sumed long term slip rate on the Ulakhan fault. However, aswas noted by Hindle et al. (2006), a backslip model of theelastic loading from the Kamchatka subduction zone sug-gests that even at distances∼1000 km away an additional∼1–2.5 mm/yr of broadly southeasterly displacement may beremoved from GPS stations due to transient northwesterly



Inya-Yama

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fault, one of the clearest lineaments morphologically, yet not associated with modern seismicity. b) Chai-Yureya

fault, possible pull-apart basin (Upper Nera) and fault trace. Despite clear, modern seismic activity, and some

evidence of recent finite offsetting of rivers, the morphological expression of the Chai-Yureya is poor. c) Trace

of the Ketanda fault, likely EU-OK boundary. d) Possible pull-apart on the Elgin-Oymayakon fault.

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Fig. 5. Examples of the morphologic expression of faults and inferred faults from satellite images.(a) Inya-Yama fault, one of the clearestlineaments morphologically, yet not associated with modern seismicity.(b) Chai-Yureya fault, possible pull-apart basin (Upper Nera) andfault trace. Despite clear, modern seismic activity, and some evidence of recent finite offsetting of rivers, the morphological expression ofthe Chai-Yureya is poor.(c) Trace of the Ketanda fault, likely EU-OK boundary.(d) Possible pull-apart on the Elgin-Oymayakon fault.

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65˚

10 mm/yr

(b)(a) Smooth extrusion GPS

Fig. 6. Shear planes (lines of zero extension) due to deformation fields from OK (red and blue lines), calculated

after the method of Holt (2000), and compared to orientations of faults (black lines) within OK a) Based

on smooth extrusion model velocity field. b) Based on GPS velocity field. The orientation of the model

shear planes in a) matches quite well, the orientation of the major strike-slip faults suggested to be controlling

deformation of the region.

20

Fig. 6. Shear planes (lines of zero extension) due to deformation fields from OK (red and blue lines), calculated after the method ofHolt(2000), and compared to orientations of faults (black lines) within OK(a) Based on smooth extrusion model velocity field.(b) Based on GPSvelocity field. The orientation of the model shear planes in(a) matches quite well, the orientation of the major strike-slip faults suggested tobe controlling deformation of the region.

pushing, which will be released over a normal seismic cycleon the OK-PA plate boundary. Adding the Kamchatka pushto the TAL1 (block 3) velocity gives a net motion crudelycorrected for the effects of the Kamchatka margin elasticloading, which falls very close to the likely long term slip rate

of the Ulakhan fault. If indeed the TAL1 and KUL1 veloc-ities are representative of long term displacement, this sug-gests an offset rate along the Chai-Yureya fault and blockboundary of∼1 mm/yr. However, this remains speculativeand it could also be that long term offset rates between blocks



are even lower than suggested by measured elastic displace-ments, and the majority of the extrusion of Northwestern OKis partitioned on the Ulakhan fault.

The long tectonic history of the region in question,linked to the Jurassic-Cretaceous inversion of Siberian plat-form margin sediments during the continental collision thatformed the Verkhoyansk fold-thrust belt (Parfenov et al.,1995), means that some of the lineaments likely substantiallypredate the initiation of the modern tectonic situation dis-cussed here. The possible offset of a dolerite body by∼8 kmalong the Chai-Yureya fault may for instance be evidence ofa much longer history of motion. Earlier positions of theEU-NA pole of rotation given byGaina et al.(2002) havesuggested the region would have been under similar com-pression to today between∼20–10 Ma for instance, whichmay correspond to earlier phases of slip. However, presentday seismic activity and finite offset of Recent features suchas fluvial systems demonstrate that a significant number ofthe faults are tectonically active today.

6 Conclusions

The possibility that Northwestern OK consists of a numberof smaller, fault-bounded blocks is fairly well-supported bymostly seismological and GPS data. Based on the relativelysparse data available, the block model suggests a highest ab-solute rate of southward escape of block 4, at the centre ofOK, with diminishing velocities to the east and west until theOK-NA or OK-EU boundary is encountered. Absolute ratesof relative motion between blocks remain undetermined, andtrends in velocity are most clear between the OK-NA bound-ary and block 4. The extrusion motion seems to continueinto the mostly aseismic Sea of Okhotsk, but probably at areduced rate, meaning a small amount of N-S or NW-SE con-traction may occur along the coast of Okhotsk today. The factthat clearly active fault systems have sometimes very littlemorphological expression, suggests either that relative sliprates between blocks are low, or that the block system hasbeen active for only a very short time.

The Ulakhan fault forming the OK-NA boundary isbounded on both sides by more or less rigid crust, whichagrees with broadly constant, geological time scale displace-ment along its length suggested by river offset data. Therelatively high magnitude of geological displacement of theUlakhan fault, currently believed to be∼5–6 mm/yr, is diffi-cult to reconcile with the lower measured motion of portionsof the interior of OK relative to NA. This may be becauseGPS velocities are measuring transient, elastic motions, andthe velocity southwards is slowed by elastic loading from theKamchatka subduction zone.

The local velocity differences suggested by the blockswithin Northwestern OK nevertheless generate a pattern ofvelocities somewhat similar to a smooth extrusion modelfor Northwestern OK. The region closest to the EU-OK-NA

triple junction behaves as if in a pure contraction deformationmode, though it is unclear what mechanism exactly leads tothis.

The analysis shows the importance of joint seismic andspace geodetic interpretation in understanding the function-ing of plate boundary zones and we suggest more geodeticdata could allow a significant test of the model we presenthere.

Acknowledgements.The authors thank their Russian colleagues,particularly V. Egorov, V. Imaev, L. Gunbina and B. Sedov for con-tinuing support and discussion of research in Northeastern Russia.DH acknowledges support of the Alexander von Humboldt Stiftung.Figures were prepared using GMTv4.0 (Wessel and Smith, 1998).We are very greatful for helpful reviews by Jeremy Hourigan andan anonymous reviewer.

References

Allmendinger, R., Smalley, R., Bevis, M., Caprio, H., and Brooks,B.: Bending the Bolivian orocline in real 295 time, Geology,33(11), 905-908, 2005.

Apel, E., Burgmann, R., Steblov, G., Vasilenko, N., King, R., andPrytkov, A.: Independent active microplate tectonics of northeastAsia from GPS velocities and block modeling, Geophysical Re-search Letters, 33, L11303, doi:10.1029/2006GL026 077, 2006.

Burgmann, R., Kogan, M., Steblov, G., Hilley, G., Levin, V., andApel, E.: Interseismic coupling and 300 asperity distributionalong the Kamchatka subduction zone., J. Geophys. Res., 110,B07405, doi:10.1029/2005JB003648, 2005.

Cook, D., Fujita, K., and McMullen, C.: Present day plate interac-tions in NE Asia: North American, Eurasian and Okhotsk Plates,J. Geodyn., 6(1–4), 33-51, 1986.

Eldholm, O., Karasik, M., and Reksnes, P. A.: The North Ameri-can plate boundary, in: the Arctic Ocean Region, edited by: A.Grantz, L. J. and Sweeney, J. F., The Geology of North America,Geological Society of America, Boulder, Colorado, L, 171-184,1990.

Fournier, M., Jolivet, L., Huchon, P., Sergeyev, K., and Oscorbin,L.: Neogene strike-slip faulting in Sakhalin and the Japan Seaopening, J. Geophys. Res., 99(B2), 2701-2725, 1994.

Fujita, K., Stone, D., Layer, P., Parfenov, L., and Kozmin, B.: Coop-erative program helps decipher tectonics of Northeastern Russia,EOS T. Am. Geophys. Un., 78(24), p. 245, 252-253, 1997.

Fujita, K., McLean, M., Mackey, K., and Kozmin, B.: The 1971Artyk earthquake: Is the locus of motion changing in northeastRussia, EOS T. Am. Geophys. Un., Supplement, 83, Abstractno. F1247, 2002.

Fujita, K., Sella, G., Mackey, K., Stein, S., Park, K.-D., and Imaev,V.: Relationships between seismicity and 315 GPS determinedvelocities in northeast Asia, EOS T. Am. Geophys. Un., 85(47)of Fall meeting suppl., Abstract no. F667, 2004.

Fujita, K., Kozmin, B., Mackey, K., Riegel, S., Imaev, V., andMcLean, M.: Seismotectonics of the Chersky seismic belt, east-ern Sakha Republic (Yakutia) and Magadan district, Russia,Stephan Mueller Spec. Publ. Ser., this volume, 2009.

Gaina, C., Roest, W., and Muller, R.: Late Cretaceous Cenozoicdeformation of northeast Asia, Earth Planet. Sc. Lett., 197(3–4),273-286, 2002.



Hindle, D., Kley, J., Stein, S., Dixon, T., and Norabuena,E.: Consistency of geologic and geodetic displacements dur-ing Andean orogenesis, Geophys. Res. Lett., 29(8), 1188,doi:10.1029/2001GL013 757, 2002.

Hindle, D., Fujita, K., and Mackey, K.: Current deformation ratesand extrusion of the northwestern Okhotsk plate, northeast Rus-sia, Geophys. Res. Lett., 33, L02306, doi:10.1029/2005GL024814, 2006.

Holt, W.: Correlated crust and mantle strain fields in Tibet, Geol-ogy, 28(1), 67-70, 2000.

Imaev, V., Imaeva, L., Kozmin, B., and Fujita, K.: Active faults andrecent geodynamics of the Yakutian seismic belts, GeotectonicsEnglish Translation, 28, 146-158, 1994.

Imaev, V., Imaeva, L. P., and Kozmin, B.: Seismotectonics of Yaku-tia, GEOS, Moscow, 1st edn., 2000 (in Russian).

Kogan, M., Burgmann, R., Vasilenko, N., Scholz, C., King, R.,Ivashchenko, A., Frolov, D., Steblov, G., Kim, C. U., and Egorov,S.: The 2000Mw 6.8 Uglegorsk earthquake and regional plateboundary deformation ofSakhalin from geodetic data, Geophys.Res. Lett., 30(3), 1102, doi:10.1029/2002GL016399, 2003.

Krylov, I.: Geological factors in the course changes of a system ofriver valleys in mountain regions (exemplified by the headwatersof the Kolyma and Indigirka rivers), Soviet Hydrology: SelectedPapers, 6, 562-565, 1971.

Levin, V., Shapiro, N., Park, J., and Ritzwoller, M.: Seismic evi-dence for catastrophic slab loss beneath Kamchatka, Nature, 418,763-765, 2002.

Mackey, K., Fujita, K., Gunbina, L., Kovalev, V., Imaev, V.,Kozmin, B., and Imaeva, L.: Seismicity of the Bering Strait re-gion: Evidence for a Bering block, Geology, 25(11), 979-982,1997.

Morgan, W. J.: Rises, trenches, great faults and crustal blocks, J.Geophys. Res., 73(6), 1959-1982, 1968.

Parfenov, L., Kozmin, B., Grinenko, O., Imaev, V., and Imaeva, L.:Geodynamics of the Chersky Seismic Belt, J. Geodyn., 9(1), 15-37, 1988.

Parfenov, L., Prokopiev, A., and Gaiduk, V.: Cretaceous frontalthrusts of the Verkhoyansk fold belt, eastern Siberia, Tectonics,14(2), 342-358, 1995.

Pedoja, K., Bourgeois, J., Pinegina, T., and Higman, B.: DoesKamchatka beong to North America? An extruding Okhotskblock suggested by coastal neotectonics of the Ozernoi Penin-sula, Kamchatka, Russia, Geology, 34(5), 353-356, 2006.

Riegel, S. A., Fujita, K., Kozmin, B. M., Imaev, V. S., and Cook,D. B.: Extrusion tectonics of the Okhotsk plate, northeast Asia.,Geophys. Res. Lett., 20(7), 607-610, 1993.

Sella, G. F., Dixon, T. H., and Mao, A.: REVEL: A model forRecent plate velocities from space geodesy, J. Geophys. Res.,107(B4), 2081, 10.1029/2000JB000033, 2002.

Seno, T., Sakurai, T., and Stein, S.: Can the Okhotsk plate be dis-criminated from the North American plate?, J. Geophys. Res.,101(B5), 11 305-11 315, 1996.

Silver, P. G.: Seismic anisotropy beneath the continents: Probingthe depths of geology, Annu. Rev. Earth Pl. Sc., 24, 385-432,1996.

Steblov, G. M., Kogan, M. G., King, R. W., Scholz, C. H.,Burgmann, R., and Frolov, D. I.: Imprint of the North Americanplate in Siberia revealed by GPS, Geophys. Res. Lett., 30(18),1924, doi:10.1029/2003GL017805, 2003.

Stein, S. and Sella, G. F.: Plate boundary zones: Concepts and ap-proaches, in: Plate Boundary Zones, AGU Geodynamics Series,edited by: Stein, S. and Freymuller, G., Washington, DC, 30,1-26, doi:10/1029/030GD01, 2002.

Wessel, P. and Smith, W. H. F.: New, improved version of GenericMapping Tools released, EOS T. Am. Geophys. Un., 79(47),579 pp., 1998.

Worrall, D., Kruglyak, V., Kunst, F., and Kuznetsov, V.: Tertiarytectonics of the Sea of Okhotsk, Russia: Far-field effects of theIndia-Eurasia collision, Tectonics, 15(4), 813-826, 1996.


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